Re: [PATCH v2 13/13] md: raid10: avoid direct access to bvec table in handle_reshape_read_error

On Thu, Mar 2, 2017 at 3:47 PM, Shaohua Li [off-list ref] wrote:

On Thu, Mar 02, 2017 at 10:37:49AM +0800, Ming Lei wrote:

quoted

Hi Shaohua,

On Wed, Mar 1, 2017 at 7:46 AM, Shaohua Li [off-list ref] wrote:

quoted

On Tue, Feb 28, 2017 at 11:41:43PM +0800, Ming Lei wrote:

quoted

The cost is 128bytes(8*16) stack space in kernel thread context, and
just use the bio helper to retrieve pages from bio.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 drivers/md/raid10.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 0b97631e3905..6ffb64ab45f8 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -4670,7 +4670,15 @@ static int handle_reshape_read_error(struct mddev *mddev,
      struct r10bio *r10b = &on_stack.r10_bio;
      int slot = 0;
      int idx = 0;
-     struct bio_vec *bvec = r10_bio->master_bio->bi_io_vec;
+     struct bio_vec *bvl;
+     struct page *pages[RESYNC_PAGES];
+
+     /*
+      * This bio is allocated in reshape_request(), and size
+      * is still RESYNC_PAGES
+      */
+     bio_for_each_segment_all(bvl, r10_bio->master_bio, idx)
+             pages[idx] = bvl->bv_page;

The reshape bio is doing IO against the memory we allocated for r10_bio, I'm
wondering why we can't get the pages from r10_bio. In this way, we don't need
access the bio_vec any more.

Reshap read is special and the bio(r10_bio->master_bio) isn't allocated from
.r10buf_pool, please see reshape_request().

Why does this matter? The bio is still doing IO to the memory allocated in
r10_bio. bi_private->r10_bio->the pages.

Good question, :-)

My guess why you think the reshape read is special is the bio->bi_private
doesn't pointer to resync_pages. And since you let resync_pages pointer to
r10_bio and not vice versa, we can't find r10_bio, correct? I think this is
another reason why r10_bio embeds resync_pages (I mean a pointer to
resync_pages). With this way I suggested, the reshape read isn't special at

We still can get the resync_pages for r10_bio->master_bio via
r10_bio->devs[0].bio in handle_reshape_read_error(), will do that in v3.

all. If we allocate memory for r10_bio/r1_bio, we shouldn't need access any bio
bvec.

Right.



Thanks,
Ming Lei